Stem anatomy at various developmental stages of secondary growth in Turbina corymbosa (Convolvulaceae)

Abstract

Background – Population growth of lianas in the tropical forest is credited to their ability of CO2 sequestration and efficiency of the narrow stems to supply water required for the amount of foliage it bears. Turbina corymbosa (L.) Raf. (Convolvulaceae Juss.) is one of the fast-growing invasive species of scrambling woody lianas. It covers trees entirely within a short period to compete with above-ground resources (particularly sunlight). However, no information is available on how it manages to cope up with an increasing demand of water supply and mineral nutrients. What are the structural and developmental patterns adapted by this species to expand the stem diameter for efficient supply of below-ground resources? Therefore, our aim was to investigate the secondary growth patterns and structure of secondary xylem and phloem in T. corymbosa.Methods – Several samples of the stem with various diameters were studied using a histological method. Morphological and anatomical analyses were carried out using light microscopy.Key results – With the initiation of secondary growth, stems lose their circular outline rapidly due to unequal deposition of secondary xylem and formation of successive cambia. New successive cambia initiate from parenchymatous cells as small crescent-shaped fragments on asymmetric/opposite sides and result in a different stem conformation. Though several segments of successive cambia are formed, very few stem samples form complete cambium rings. The secondary xylem formed by successive cambia is diffuse porous with indistinct growth rings and is composed of both wide and narrow (fibriform) vessels, tracheids, fibres, axial and ray parenchyma cells. The secondary phloem consists of sieve tube elements, companion cells, axial and ray parenchyma cells. In fully grown plants, cambial action (internal cambium) occurrs between the intraxylary phloem and protoxylem and produces secondary xylem and phloem near the pith region.Conclusion – Structural alterations and unequal deposition of conducting elements, occurrence of intraxylary phloem and flattening of the stem are suggested to facilitate rapid growth of the plants by providing required minerals and nutrients. Internal cambium formed at the periphery of the pith is bidirectional and produces secondary xylem externally and intraxylary phloem internally. Continued development of intraxylary phloem from the internal cambium provides an additional path for rapid and safe translocation of photosynthates.

Formation of medullary phloem in Argyreia nervosa (Burm. f.) Bojer

“…Formation of intraxylary phloem is universal in Convolvulaceae and in most of the members, internal cambium initiates at the pith margin i.e. centrifugally to the intraxylary phloem (8,(11)(12)(13)17,18,25,26,31). In the present study, also observed periclinal divisions at the marginal pith cells between protoxylem and intraxylary phloem and results into a weak cambium (i.e.…”

“…Though, these features are common in most of the species of the family, formation of successive cambia lack in members viz., Convolvulus, Cressa and Wilsonia. On the other hand, occurrence of intraxylary/medullary/internal phloem is constant feature in all the members of the family studied so far (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18). In the present study, Argyreia nervosa (Burm.…”

“…Stem anatomy of the family Convolvulaceae is unique and shows various types of variant growth patterns such as development of successive cambia, interxylary phloem, intraxylary phloem, internal cambium, successive internal cambia or presence of medullary bundles (2,3,7,(10)(11)(12)(16)(17)(18). Though, these features are common in most of the species of the family, formation of successive cambia lack in members viz., Convolvulus, Cressa and Wilsonia.…”

“…The internal cambium may be functionally unidirectional (producing only phloem) or bidirectional i.e. produce xylem centripetally and phloem centrifugally (12,13,18,31). Handa (32) reported a unique case of unidirectional cambium in Hoya carnosa and Marsdenia tomentosa (s.f.…”

“…Climbing habit in the family Convolvulaceae has evolved with various kinds of cambial variants such as successive cambia, internal cambium, successive internal cambia, functionally inverse cambium, interxylary (included phloem), intraxylary (internal) phloem, medullary bundles etc. (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18). Occurrence of above-mentioned anatomical features are common throughout the family except presence of medullary bundles in the Convolvulaceae is reported in Argyreia nervosa by Pant and Bhatnagar (10) and in A. nervosa and A. roxburghii by Govil (19).…”